The current generation of adenoviral vectors for gene therapy contains deletions of the early region 1 (“E1”), where new genetic information can be introduced. The E1 deletion renders the recombinant virus replication defective. It was generally thought that E1-deleted vectors would not express any other adenoviral genes, because E1 is reported to trigger the transcription of the other adenoviral genes. It has been shown by us and others that these vectors express several early (e.g., E2A) and late genes (e.g., fiber and penton-base) in the absence of E1. This means that delivery of a therapeutic gene using E1-deleted adenoviral vectors will result in expression of the therapeutic protein and adenoviral proteins. A cytotoxic immune response is evoked against such transduced cells. It has been shown that cytotoxic T-lymphocytes (“CTLs”) directed against both the transgene product and products encoded by the vector are activated, following vector administration into immunocompetent animals (Song et al., Hum. Gene Ther. 8:1207, 1997; Yang et al., J. Virol. 70:7209, 1996). Activated CTLs subsequently eradicate transduced cells from the recipient. Consistent with this, the longevity of transgene expression is significantly extended in immuno-deficient and immuno-compromised animals.
Expression of at least some adenoviral genes in a target cell is at least in part due to background replication of the recombinant adenoviral vector genome and/or background activity of promoters driving the respective adenoviral genes (Yang et al., Nature Genet. 7:362, 1994; Lusky et al., J. Virol. 72:2022, 1998). As a result of the expression of at least some adenovirus proteins in a target cell in a recipient, an immune response may be mounted against transduced cells. Such an immune response is often not desired, especially when long-term expression of a transgene is aimed for. One mechanism by which adenovirus proteins in a target cell in a recipient may cause the immune system of the recipient to remove the target cell is the following. Proteins encoded by expressed adenovirus genes can be processed into small peptides in a proteosome of the target cell. Peptides produced during this processing can subsequently be presented at the cell surface of the transduced cells in the complex of MHC class-I and □b2-microglobulin molecules. Finally, one or more of the peptides may be recognized as non-self peptides by circulating CTLs whereupon transduced cells can be eradicated from the recipient (reviewed in Ploegh, Science 280:248, 1998).